The present invention relates to an anti-overvoltage protector of a gas-carrying discharge tube type which is intended to protect electric appliances, etc. connected to an electric line against, for example, a high surge voltage and, more particularly, to an anti-overvoltage protector which is equipped with trigger electrodes for improving the delay in the discharge operation of the anti-overvoltage protector which occurs when an overvoltage has been applied thereto.
In order to protect electric appliances, etc. connected to an electric line against, or from, the high surge voltage or the like induced thereinto, an anti-overvoltage protector like a gas-carrying discharge tube is disposed between the electric line and the earth. In this type of anti-overvoltage protector, when a high surge voltage has been applied thereto, a time delay in the commencement of its discharge operation would result in the failure to protect the electric appliances against such surge voltage, in some cases. To prevent such time delay in the commencement of the discharge operation, for example, U.S. Pat. No. 3,588,576 discloses in its specification an anti-overvoltage protector of the type wherein a trigger discharge gap having a concentrated potential gradient is formed in the vicinity of the main electrodes effecting the discharge operation of the protector; and when a high surge voltage or the like has been applied to the protector, the resultant discharge operation in that trigger discharge gap causes a concentrated potential gradient to occur in the vicinity of the main electrodes, thereby hastening the commencement of the discharge which occurs between the main electrodes. The anti-overvoltage protector disclosed in the U.S. patent specification is one which comprises a hollow cylindrical member formed of insulating material such as ceramics, glass, etc., a pair of main electrodes hermetically mounted onto both end openings of the hollow cylindrical member so as to form a closed chamber therein and having their discharge surfaces opposed, within the hollow cylindrical member, to each other with a gap interposed therebetween, and a slender conductive layer extending, within an ionizing region of said gap, along the inner wall surface of the hollow cylindrical member and axially of this member. In this anti-overvoltage protector, when a high surge voltage is applied thereto, an electric field is concentrated in the trigger discharge gap between the end of the slender conductive layer and its opposed electrode, whereby the gas existing in the ionizing region of the discharge gap between the main electrodes is ionized, thereby hastening the commencement of the discharge between the main electrodes. The slender conductive layer of the anti-overvoltage protector disclosed in the U.S. Patent Specification is disposed such that it is conductively connected to one of the main electrodes and extends to the other of the main electrodes and this extension terminates in such a manner that it opposes the other main electrode with a gap greater than the discharge gap between the main electrodes, interposed therebetween. Further, for example, U.S. Pat. No. 3,989,985 discloses in its specification another anti-overvoltage protector which is arranged so as to hasten the commencement of the discharge between the main electrodes by using a slender conductive layer similar to that which is disclosed in the U.S. Pat. No. 3,588,576. This other anti-overvoltage protector is the same as the one which is disclosed in the U.S. Pat. No. 3,588,576 excepting that the slender conductive layer is conductively connected to one of the main electrodes and extends to the other of the main electrodes beyond the discharge gap between the main electrodes and this extended portion opposes the other of the main electrodes with a gap greater than the discharge gap between the main electrodes, interposed between both. However, it is extremely difficult to form with high precision the slender conductive layer disclosed in these U.S. patent specifications, on the inner wall surface of the hollow cylindrical member in such a manner that the gap between the slender conductive layer and its opposed main electrode is greater than the discharge gap between the main electrodes. Further, since a limitation is imposed upon the precision with which various parts or members constituting the anti-overvoltage protector are fabricated, limitation is also imposed upon the dimensional accuracy of the discharge gap. For example, it is extremely difficult to form a discharge gap of 0.3 mm or less with high accuracy. For this reason, there arises the problems that the surge voltage response characteristic of the anti-overvoltage protector deteriorates, or variations in the discharge characteristic are liable to be produced.
Further, the following anti-overvoltage protector is known as an anti-overvoltage protector having a trigger discharge gap, other than the ones which have been mentioned above. That is to say, a recess is provided in each discharge surface of a pair of main electrodes opposing each other, and an insulating bar formed of, for example, ceramic material is provided in such a manner that it is fixedly clamped, in the recess, between the main electrodes, and an annular groove having a width smaller than the discharge gap between the main electrodes is provided in the circumferential surface of a substantially central portion of its axial length, and a conductive layer which is conductively connected to the main electrodes is applied onto the whole outer surface of the insulating bar excluding that which corresponds to the annular groove, whereby the commencement of the discharge between the main electrodes is promoted by using the annular groove as a trigger discharge gap.
In the above-mentioned anti-overvoltage protector wherein the narrow annular groove provided in the outer surface of the insulating bar is used as the trigger discharge gap, since the insulating bar is formed of an insulating material having high heat resistance such as ceramics, the use of a special fabricator such as, for example, a laser fabricator is required to form such a narrow annular groove. Further, since the annular groove has a very narrow width, when forming the conductive layer on the whole outer surface portion of the insulating bar excluding the surface portion which corresponds to the annular groove, the conductive layer is formed on this groove as well. For this reason, sufficient care is required to be taken so as not to cause the trigger discharge gap to be shortcircuited. Accordingly, the working efficiency is decreased and the cost involved is increased.